This invention is directed to the scanning devices, such as bar code scanners, and is more particularly concerned with an arrangement for reading optical symbols, e.g., small linear or two-dimensional bar code symbols that are directly marked on parts or components. Such bar code symbols may be used to identify the part number and serial number of an electronic microcircuit devices, communications cables and connectors, diesel engine ignitors, as well as many other products. The invention is more particularly concerned with a device that is combined with an illuminator for shining light on the bar code symbol or other symbol which is marked onto a specularly reflective, i.e., mirrored surface.
Within the past several years, two dimension bar code systems have begun to be employed on small articles so that the article can be tracked thought a manufacturing process. For example, two-dimensional bar code symbols are inscribed onto electronic parts, e.g. as miniature etched or printed part numbers and serial numbers on microprocessors, other integrated circuits, and printed circuit boards. Because of the high information density of the 2D symbols, these can carry the part number, part serial number, and manufacturing history of the part, such as the place and date of manufacture, and all in a square that may typically be only three millimeters on a side. The symbols can be screen printed, ink jet printed, or laser-etched directly onto the top surface or the case of the device or part. However, in order to read the bar code easily on the first attempt, the conventional wisdom is that the illumination should impinge on the bar code symbol in a way that fully illuminates it but avoids flare from specular reflection.
Because of the small size of 2-D bar code symbols, the symbols can be difficult for the scanner to interpret where the lighting is weak or if the contrast is too low. For example, Datamatrix™ bar code, which has been selected as the EIA (Electronics Industry Association) standard for small parts marking, is in the form of a matrix of bars about three millimeters on a side. These are inscribed by laser etching onto the plastic or ceramic housing or package for the electronic module, or can be printed by an ink jet technique. In either case, if the lighting is such that the scanner picks up specular reflection, the bar code symbol typically produces a wash out, and may take several attempts to obtain a valid reading. Consequently, current bar code scanners have had some difficulty in obtaining bar code data in industrial processes where the small parts are to be identified from these very small bar code symbols.
In addition to this, many times it is necessary to imprint the bar coded information directly onto a flat shiny surface, e.g., the surface of a semiconductor chip, the polished steel shaft of an automotive part, or a plated surface of industrial device, such as a nuclear fuel rod, a oil-drilling bit, or any of many other parts or devices. In these cases, the light typically is incident at a low angle and the scanner is held with its optic axis perpendicular to the specular surface. This directs most of the light that is incident on the mirrored surface so it is reflected away from the scanner, such that these highly reflective areas appear as a relatively dark areas on the image of the bar code symbol. The light impinging on the printed or etched portions of the symbol are diffused, i.e., scattered in all directions, so that some of the light is visible in the scanner, and appears as the light portions of the symbol. Because only a small portion of the diffused light reaches the scanner, the symbol is made white, i.e., printed in white ink, if possible, for maximum reflectance. However, it is not always possible to use make the symbol white or use a highly reflective diffuse coating, especially when the bar code symbol is etched, engraved, or peened directly onto the mirrored surface of a component or part. Consequently, these direct marked symbols have been difficult to read for imaging bar code scanners. Of course the situation would also be the same with bar coded symbols in which a the symbol is formed of a mirror-finish material printed onto a diffusive surface.